A bolometer is a device designed to measure the intensity of radiation, usually situated in the infrared, to which it is subjected, by transforming the energy of this radiation into thermal energy. The resultant heating of the bolometer causes the variation of an electric variable such as the electrical resistance of a conductor connected to a circuit outside the bolometer. In the case for example of a detector comprising a microbolometer matrix, this electric circuit, referred to as “readout” circuit, performs the functions of matrix addressing and reading stimuli sent to each microbolometer, and converts the resultant signals to a format that can be used in particular for imaging (for example in the form of a video signal). In order to obtain the best possible performances, the microbolometers are operated under a relatively low gas pressure (or under moderate pressure of a gas with low thermal conductivity), in order for the thermal dissipation due to this gas to be negligible vis-à-vis the intrinsic thermal conductance of the microbolometers.
The readout circuit measures the relative variation of said electric variable (attached to the bolometer's sensitive element) which depends on the temperature. In the case of non-cooled detectors (which are simpler and less expensive than detectors equipped with a cooling system), the bolometer's temperature variation is for its part proportional to the power of radiation received, the proportionality constant (called “thermal resistance” and which we shall designate Rthb) usually being between 5·106 and 2˜107 K/W.
FIG. 1 relates to a standard microbolometer with a useful surface area of 40 μm×30 μm, having a thermal resistance Rthb equal to 107 K/W and unit absorption, placed at the focus of an optical system having an aperture angle of 53° equipped with a spectral filter offering a constant transmission equal to 0.9 in the infrared (more precisely, within a range of wavelength between 8 and 14 μm). The curves represent the radiation power received and the increase in temperature of the microbolometer as a function of the temperature of the radiation source, regarded as a “black body”. This heating takes place even if the imaging system is not in operation (i.e. in the absence of electrical stimuli).
For a microbolometer provided with a “perfect” spectral filter, i.e. having zero transmission for all the wavelengths outside the abovementioned range, the increase in the microbolometer's equilibrium temperature with the temperature of the source (solid curve) is linear once the latter exceeds 2000 K. For example, the static observation of a source such as the sun (approximately 6000 K) causes an increase of the order of 100 K in the temperature of the microbolometer.
In practice, the spectral filters, even those of good quality, installed in infrared imaging systems are not perfect, in that they allow the passage of a low, but not zero, power of radiation at wavelengths situated outside the filter's theoretical operating range. Very hot sources however emit much more power in the visible range than in the infrared. Therefore, the optical power received on the detector, outside the filter's infrared theoretical range, can be considerable, and even, on occasion, preponderant with a very high source temperature and/or for filters of mediocre quality. In this case, the heating estimate indicated previously can be considerably lower than the actual value. The dashed curve in FIG. 1 shows the effective variation in the temperature of the microbolometer when it is provided with a “non-perfect” filter, having a transmission equal to 10−3 outside the abovementioned infrared range. Of course, the use of higher thermal resistances, advantageous under normal conditions of use since they increase the microbolometer's sensitivity, means even greater heating.
Microbolometers are usually designed to operate, and this is one of their advantages, close to the ambient temperature. But these microbolometers are constituted by materials (such as vanadium oxides or amorphous silicon) which exhibit a permanent, or at least durable change in their electrical characteristics (and also possibly mechanical deformation of their structure) for such rises in temperature, even temporary. Very high illuminations can even lead to their physical destruction.
Moreover, even if temporarily over-illuminated microbolometers are not destroyed, a change, even temporary, in the electrical resistance values of the image points (probable or inevitable beyond 100 to 200 K heating, for most of the microbolometers) renders inoperative the “offset” compensation electrical device, usually integrated in the readout circuit, which has been calibrated for the original spatial distribution of the individual resistances on the microbolometer.
Consequently, the observation, even transient (over a period of the order of the microbolometer's thermal time constant, i.e. usually from some milliseconds to some tens of milliseconds) of very hot sources, for example the filament of an incandescent lamp (approximately 3000 K), or the sun, is usually fatal for this type of device.
As imaging systems based on detectors of the bolometric type according to the state of the art are not compatible with the observation, even temporary or accidental, of very intense sources, it is necessary to limit the use of these systems to environments that are essentially not very aggressive, or to take constraining precautions during the use of such systems.